What does the word postmark mean?

Usage examples for postmark

  1. Dollie turned to him, recognising a menace in the words, and took the paper which bore the postmark from his hands. – The Chase of the Golden Plate by Jacques Futrelle
  2. Then came a thick envelope on which blazed the Chicago postmark. – The Sherrods by George Barr McCutcheon
  3. I wish to Heaven I'd thought of that postmark! – Kenny by Leona Dalrymple